Circuit substrate and semiconductor device

ABSTRACT

To improve a TCT characteristic of a circuit substrate. The circuit substrate comprises a ceramic substrate including a first and second surfaces, and first and second metal plates respectively bonded to the first and second surfaces via first and second bonding layers. A three-point bending strength of the ceramic substrate is 500 MPa or more. At least one of L1/H1 of a first protruding portion of the first bonding layer and L2/H2 of a second protruding portion of the second bonding layer is 0.5 or more and 3.0 or less. At least one of an average value of first Vickers hardnesses of 10 places of the first protruding portion and an average value of second Vickers hardnesses of 10 places of the second protruding portion is 250 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior International ApplicationNo. PCT/JP2016/003531, filed on Aug. 1, 2016 which is based upon andclaims the benefit of priority from Japanese Patent Application No.2015-189990, filed on Sep. 28, 2015; the entire contents of all of whichare incorporated herein by reference.

FIELD

Embodiments described herein generally relate to a circuit substrate anda semiconductor device.

BACKGROUND

In recent years, as industrial equipment comes to have higherperformance, a power module to be mounted thereon is being made to havea higher output. With the above, a semiconductor element is being madeto have a higher output. An operation guaranteed temperature of thesemiconductor element is 125° C. to 150° C., but there is a possibilitythat the operation guaranteed temperature is raised to 175° C. or morein the future.

With rise of the operation guaranteed temperature of the semiconductorelement, a high TCT characteristic is demanded of a ceramic metalcircuit board. TCT stands for a thermal cycle test. The TCT is a methodof measuring durability of the ceramic metal circuit board by carryingout a process of changing a temperature in order of low temperature→roomtemperature→high temperature→room temperature as one cycle.

A conventional ceramic metal circuit substrate has a ceramic substrateand a metal plate. The metal plate is bonded to the ceramic substratevia a bonding layer formed by using a brazing material. The bondinglayer has a protruding portion which extends onto the ceramic substratein a manner to protrude from between the ceramic substrate and the metalplate. The above-described ceramic metal circuit substrate has a highdurability to the TCT of 5000 cycles. By reducing a gap of theprotruding portion, the TCT characteristic can be improved. However,when the operation guaranteed temperature is 175° C. or more,improvement of the TCT characteristic by merely removing the gap of theprotruding portion is limited.

A power density indicating performance of a power module is obtained bythe following formula.power density=(rated current×rated voltage×number of semiconductorelements mounted on module/volume of module

The power density can be made large by increasing the number of thesemiconductor elements mounted on the module or decreasing the volume ofthe module, for example. In order to improve these two parameters, it isrequired to mount a plurality of semiconductor elements on a ceramicmetal circuit board. In order to realize a ceramic metal circuit boardon which a plurality of semiconductor elements can be mounted and whichis small in volume, it is preferable to narrow an arrangement intervalbetween a plurality of metal plates. Also in a case of narrowing thearrangement interval between the metal plates, improvement of a TCTcharacteristic at a high temperature of 175° C. or more is required ofthe ceramic metal circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic diagram illustrating aconfiguration example of a circuit substrate.

FIG. 2 is a cross-sectional schematic diagram illustrating anotherconfiguration example of the circuit substrate.

FIG. 3 is an enlarged diagram illustrating a part of the circuitsubstrate illustrated in FIG. 2.

FIG. 4 is a chart illustrating a loading condition.

DETAILED DESCRIPTION

A circuit substrate according to an embodiment comprises: a ceramicsubstrate including a first surface and a second surface; a first metalplate bonded to the first surface via a first bonding layer; and asecond metal plate bonded to the second surface via a second bondinglayer. The first bonding layer has a first portion protruding frombetween the first surface and the first metal plate and extending ontothe first surface. The second bonding layer has a second protrudingportion protruding from between the second surface and the second metalplate and extending onto a third surface. A three-point bending strengthof the ceramic substrate is 500 MPa or more. At least one of a ratioL1/H1 of a length L1 (μm) of the first portion to a thickness H1 (μm)thereof or a ratio L2/H2 of a length L2 (μm) of the second portion to athickness H2 (μm) thereof is 0.5 or more and 3.0 or less. At least oneof an average value of Vickers hardnesses of 10 places of the firstportion or an average value of Vickers hardnesses of 10 places of thesecond portion is 250 or less.

FIG. 1 is a cross-sectional schematic diagram illustrating aconfiguration example of a circuit substrate. The circuit substrate 1illustrated in FIG. 1 has a ceramic substrate 2, a metal plate (frontmetal plate) 3, a metal plate 4 (back metal plate), a bonding layer 5,and a bonding layer 7. The bonding layer 5 has a protruding portion 6.The bonding layer 7 has a protruding portion 8.

A three-point bending strength of the ceramic substrate 2 is 500 MPa ormore. By using the ceramic substrate having the three-point bendingstrength of 500 MPa or more, a thickness of the ceramic substrate 2 canbe reduced to 0.4 mm or less. If the ceramic substrate having athree-point bending strength of less than 500 MPa is thinned to 0.4 mmor less, a TCT characteristic is reduced. In particular, durability whena temperature on a high temperature side in a TCT is 175° C. or more isreduced.

As the ceramic substrate having the three-point bending strength of 500MPa or more, it is preferable to use a silicon nitride substrate.Three-point bending strengths of an aluminum nitride substrate and analumina substrate are about 300 to 450 MPa in general. In contrast, thesilicon nitride substrate has a high three-point bending strength of 500MPa or more, and further, 600 MPa or more. The three-point bendingstrength is measured, for example, based on JIS-R-1601.

The silicon nitride substrate has a heat conductivity of 50 W/m·K ormore, and further, 80 W/m·K or more. The heat conductivity is measured,for example, based on JIS-R-1611. A recent silicon nitride substrate hasboth high strength and high heat conductivity. The silicon nitridesubstrate having the three-point bending strength of 500 MPa or more andthe heat conductivity of 80 W/m·K or more enables reduction of athickness of the ceramic substrate 2 to 0.30 mm or less. Note that asthe ceramic substrate 2, it is possible to use not only the siliconnitride substrate but also an aluminum nitride substrate, an aluminasubstrate, an alumina substrate containing zirconia, and so on whichhave high strengths.

The metal plate 3 is a metal circuit board for mounting a semiconductorelement thereon. The metal plate 3 is bonded to a first surface of theceramic substrate 2 via the bonding layer 5. The metal plate 4 is a heatsink plate. The metal plate 4 is bonded to a second surface of theceramic substrate 2 via the bonding layer 7.

As the metal plates 3, 4, metal plates containing copper or aluminum oran alloy whose main component is copper or aluminum can be used. Themetal plate of the above-described material, since being low inelectrical resistance, is easily usable as a circuit substrate, forexample. Further, a heat conductivity of copper is as high as 398 W/m·Kand a heat conductivity of aluminum is as high as 237 W/m·K. Thus, heatrelease performance can also be improved.

Thicknesses of the metal plates 3, 4 having the above-describedcharacteristics are preferable to be 0.6 mm or more, and further, 0.8 mmor more. Increasing the thickness of the metal plate can achieve bothsecuring of a current-carrying capacity and improvement of heat releaseperformance. Further, heat can be diffused not only in a thicknessdirection of the metal plate but also in a surface direction, so thatthe heat release performance can be improved. The thicknesses of themetal plates 3, 4 are preferable to be 5 mm or less. The thickness over5 mm excessively increases a weight of the metal plate, resulting inapprehension that adjustment of sizes of the protruding portions 6, 8becomes difficult.

The bonding layer 5 bonds the first surface of the ceramic substrate 2and the metal plate 3. The bonding layer 6 bonds the second surface ofthe ceramic substrate 2 and the metal plate 4. The protruding portion 6extends onto the first surface of the ceramic substrate 2 in a manner toprotrude from between the first surface and the metal plate 3. Theprotruding portion 8 extends onto the second surface of the ceramicsubstrate 2 in a manner to protrude from between the second surface andthe metal plate 4.

The bonding layers 5, 7 are formed by applying brazing material pasteonto the ceramic substrate 2. On this occasion, if the metal plates 3, 4are thick, the brazing material paste spreads too much due to weights ofthe metal plates 3, 4. Too much spreading of the brazing material pasteincreases a brazing material amount to be removed by an etchingprocessing or the like after bonding, to cause a cost increase.Therefore, the thicknesses of the metal plates 3, 4 are preferable to be5 mm or less, and further, 3 mm or less.

When the metal plates 3, 4 are copper plates or aluminum plates, thebonding layers 5, 7 are preferable to contain at least one elementselected from Ag (silver), Cu (copper), and Al (aluminum) as a maincomponent. Further, the bonding layers 5, 7 are preferable to furthercontain at least one element selected from Ti (titanium), Hf (hafnium),Zr (zirconium), Si (silicon), and Mg (magnesium). For example, thebonding layers 5, 7 may contain Ag, Cu, and at least one elementselected from Ti, Zr, and Hf. Further, the bonding layers 5, 7 maycontain Al and at least one element selected from Si and Mg. The bondinglayer whose main component is Ag or Cu is suitable for bonding a copperplate. The copper plate has a higher heat conductivity compared with analuminum plate, and is effective in improving heat release performance.Further, the bonding layers 5, 7 are preferable to contain at least oneelement selected from In (indium), Sn (tin), and C (carbon).

At least one of a ratio L1/H1 of a protruding length L1 (μm) to athickness H1 (μm) of the protruding portion 6 and a ratio L2/H2 of aprotruding length L2 (μm) to a thickness H2 (μm) of the protrudingportion 8 is 0.5 or more and 3.0 or less. At least one of L1/H1 andL2/H2 being 0.5 or more and 3.0 or less indicates that the protrudinglength (width) is 0.5 times or more and 3.0 times or less the thickness.

In a TCT, a heat stress is applied to the protruding portions 6, 8. Theheat stress becomes larger as a temperature difference between a lowtemperature side and a high temperature side of the TCT spreads. Theprotruding lengths of the protruding portions 6, 8 are small when L1/H1and L2/H2 are less than 0.5, and thus the heat stress applied to theprotruding portions 6, 8 becomes large in the thickness direction. Theabove causes generation of a crack inside the ceramic substrate 2 andthe bonding layers 5, 7 due to the heat stress. When L1/H1 and L2/H2 areover 3.0, the heat stress applied to the protruding portions 6, 8becomes large in a protruding length direction. The above causesgeneration of a crack inside the ceramic substrate 2 and the bondinglayers 5, 7 due to the heat stress.

The heat stress is generated by shrinkage and expansion occurring due tothe temperature difference. In order to alleviate this heat stress, itis preferable to control L1/H1 and L2/H2 to within a range of 0.5 to3.0. By controlling L1/H1 and L2/H2, homogeneity in a direction to whichthe heat stress (shrinkage and expansion) is applied can be increased,and thus, the crack occurring inside the ceramic substrate 2 and thebonding layers 5, 7 can be suppressed. Consequently, the TCTcharacteristic can be improved. In particular, when the thicknesses ofthe metal plates 3, 4 are 0.6 mm or more, control of the direction towhich the heat stress is applied is effective in improving the TCTcharacteristic. L1/H1 and L2/H2 are more preferable to be 1.0 or moreand 2.0 or less.

It is preferable that at least one of average values of Vickershardnesses of arbitrary 10 places of the protruding portion 6 and ofVickers hardnesses of arbitrary 10 places of the protruding portion 8 is250 or less. When the average value of the Vickers hardnesses is over250, the protruding portions 6, 8 become too hard to obtain alleviationeffects of heat stress sufficiently. Further, the average value of theVickers hardnesses is preferable to be 90 or more and 230 or less, andfurther, 100 or more and 170 or less.

When the average value of the Vickers hardnesses of 10 places is lessthan 90, the hardness of the protruding portion is insufficient. Thereis apprehension that excessive softness may make a deformation amountdue to the heat stress larger, contrarily. The large deformation amountmay bring about deterioration of the TCT characteristic.

It is preferable that at least one of a difference between a maximumvalue and a minimum value of Vickers hardnesses of the arbitrary 10places of the protruding portion 6 and a difference between a maximumvalue and a minimum value of Vickers hardnesses of the arbitrary 10places of the protruding portion 8 is 50 or less. Large variation ofVickers hardnesses generates partial variation of deformation amountsdue to the heat stress. Thus, the variation of Vickers hardnesses ispreferable to be small.

The Vickers hardness is measured based on JIS-R-1610. A load by anindenter is applied to the protruding portions 6, 8 at a load of 50 gffor a load holding time of 10 seconds. The Vickers hardness is obtainedfrom diagonal line lengths in 2 directions of an indentation.

When measurement is difficult due to the metal plates 3, 4, the Vickershardness is measured by using a nano indenter. If the nano indenter isused, the Vickers hardness can be measured without removing the metalplate. As the nano indenter, a nano indenter manufactured by Hysitron,Inc. can be used, for example. As the indenter, a Berkovich type diamondtriangular pyramid indenter can be used, for example. With a maximumload being set to 1500 μN (micro Newton), an indentation depth at a timethat the load is gradually increased over 50 seconds is measured, and anano indentation hardness HIT is obtained. The nano indentation hardnessHIT is converted into a Vickers hardness HV by using the followingformula. This conversion formula is presented in a literature “MetalVol. 78 (2008) No. 9, P. 885 to 892”.Vickers hardness HV (kgf/mm²)=76.23×nano indentation hardness HIT(GPa)+6.3

As described above, the circuit substrate of the embodiment is a circuitsubstrate obtained by bonding the ceramic substrate having thethree-point bending strength of 500 MPa or more and the metal plates onboth surfaces thereon via the bonding layers. The bonding layer has theprotruding portion, and the ratio of the protruding length to thethickness of the protruding portion is 0.5 or more and 3.0 or less, andthe average value of the Vickers hardnesses of 10 places of theprotruding portion is 250 or less.

A plurality of metal plates may be bonded to at least one of thesurfaces of the ceramic substrate 2. FIG. 2 is a cross-sectionalschematic diagram illustrating another configuration example of thecircuit substrate. A circuit substrate 1 illustrated in FIG. 2 isdifferent compared with the circuit substrate 1 illustrated in FIG. 1 atleast in configuration where a metal plate 3 a and a metal plate 3 b areincluded as the plurality of metal plates 3. Note that the circuitsubstrate 1 may have three or more metal plates as the plurality ofmetal plates 3. Note that regarding explanation of the sameconfiguration as that of the circuit substrate 1 illustrated in FIG. 1,explanation of the circuit substrate 1 illustrated in FIG. 1 isappropriately cited.

The metal plate 3 a is bonded to a first surface of a ceramic substrate2 via a bonding layer 5 a. The metal plate 3 b is bonded to the firstsurface of the ceramic substrate 2 via a bonding layer 5 b. Regardingexplanation of the bonding layer 5 a and the bonding layer 5 b,explanation of the bonding layer 5 is appropriately cited.

FIG. 3 is an enlarged diagram illustrating a part of the circuitsubstrate 1 illustrated in FIG. 2. The bonding layer 5 a has aprotruding portion 6 a extending onto the first surface of the ceramicsubstrate 2 in a manner to protrude from between the first surface andthe metal plate 3 a. The bonding layer 5 b has a protruding portion 6 bextending onto the first surface of the ceramic substrate 2 in a mannerto protrude from between the first surface and the metal plate 3 b.

In a cross section including a thickness direction of the metal plates3, a side surface 31 a of the metal plate 3 is inclined toward the edgeof an outer surface 32 a of the metal plate 3. A side surface 31 b ofthe metal plate 3 is inclined toward the edge of an outer surface 32 bof the metal plate 3. An angle θ made by the side surface 31 a and ainner surface 33 a of the metal plate 3 a and an angle θ made by theside surface 31 b and a inner surface 33 b of the metal plate 3 b arepreferable to be 40 degrees or more and 84 degrees or less. By incliningthe side surfaces 31 a, 31 b, a heat stress can be alleviated. An anglemade by the side surface 31 a and the outer surface 32 a of the metalplate 3 a and an angle made by the side surface 31 b and the outersurface 32 b of the metal plate 3 b are preferable to be 85 degrees ormore and 95 degrees or less (perpendicular or almost perpendicular). Astructure illustrated in FIG. 3 is effective in particular whenthicknesses of the metal plates 3 a, 3 b are 0.6 mm or more, andfurther, 0.8 mm or more.

As a result of setting the angle θ of the side surface 31 a of the metalplate 3 a and the angle θ of the side surface 31 b of the metal plate 3b to 40 degrees or more and 84 degrees or less and setting the anglemade by the side surface and the outer surface of the metal plate 3 aand the angle made by the side surface and the outer surface of themetal plate 3 b to 85 degrees or more and 95 degrees or less, it ispossible to increase areas of flat surfaces of the metal plates 3 a, 3 bwhile alleviating a stress. Increasing the areas of the flat surfaces ofthe metal plates 3 a, 3 b can broaden a mountable area for asemiconductor element. When the mountable area for the semiconductorelement can be broadened, flexibility of circuit design can be improved.

The protruding length of the protruding portion is obtained with acontact portion of the side surface of the metal plate and theprotruding portion being a reference. The thickness of the protrudingportion is a thickness of the thickest part of the protruding portion.For example, it is preferable that a thickness of the protruding portion6 a at a contact portion of the side surface 31 a and the protrudingportion 6 a and a thickness of the protruding portion 6 b at a contactportion of the side surface 31 b and the protruding portion 6 b arethickest, that the protruding portion 6 a has a thickness gradient ofbecoming gradually thinner from the contact portion of the side surface31 a and the protruding portion 6 a toward an end portion, and that theprotruding portion 6 b has a thickness gradient of becoming graduallythinner from the contact portion of the side surface 31 b and theprotruding portion 6 b toward an end portion. As a result of having thethickness gradient, deformation of the protruding portion due to a heatstress can be suppressed.

A protruding length L1 of the protruding portions 6 a, 6 b is preferableto be 40 μm or less, and further, 25 μm or less. By controlling L1/H1 ofthe protruding portions 6 a, 6 b and a Vickers hardness HV, a stressalleviation effect can be improved. In other words, even with aprotruding portion having a protruding length as short as 40 μm or less,a sufficient stress alleviation effect can be obtained.

In a case where the side surfaces 31 a, 31 b are inclined, as a resultthat a shortest distance P (shortest distance from the contact portionof the side surface 31 a and the protruding portion 6 a and the contactportion of the side surface 31 b and the protruding portion 6 b) betweenskirts of inclined surfaces is narrowed to 1.1 mm or less, and further,1.0 mm or less, it becomes possible to make a circuit substrate smallerwithout decreasing the mountable area for a semiconductor element.Therefore, the circuit substrate illustrated in FIG. 3 is effective in acase where a plurality of metal plates are bonded to a first surface ofa ceramic substrate 2. A minimum value of the shortest distance P ispreferable to be 0.6 mm or more. In a case of a power module having ametal plate of 0.6 mm or more in thickness, conductivity failure mayoccur if the shortest distance P is less than 0.6 mm, considering arated voltage.

The above-described circuit substrate has a high TCT characteristic. ATCT is carried out, with one cycle being −40° C.×holding for 30minutes→room temperature×holding for 10 minutes→175° C.×holding for 30minutes→room temperature×holding for 10 minutes, for example, bymeasuring the number of the cycles where a failure occurs in the circuitsubstrate. The failure of the circuit substrate means, for example,peeling of the bonding layers 5 (5 a, 5 b), 7, a crack of the ceramicsubstrate 2, or the like.

The circuit substrate of this embodiment can exhibit an excellent TCTcharacteristic even if a holding temperature on a high temperature sidein the TCT is set to 175° C. or higher. The holding temperature of 175°C. or higher means, for example, 200° C. to 250° C. In a semiconductorelement such as a SiC element and a GaN element, a junction temperatureis estimated to become 200 to 250° C. The junction temperaturecorresponds to an operation guaranteed temperature of the semiconductorelement. Thus, durability at a high temperature is required also of thecircuit substrate.

In the circuit substrate of this embodiment, a ratio of the protrudinglength to the thickness of the protruding portion, and the Vickershardness are controlled. Thereby, the TCT characteristic can be improvedeven in a case of the circuit substrate in which a plurality of metalplates are bonded. In particular, it is possible to make the protrudinglength of the protruding portion as small as 40 μm or less, and further,25 μm or less. Therefore, the excellent TCT characteristic can beobtained even if the shortest distance P is narrowed to 1.1 mm or less,and further, 1.0 mm or less. Further, improvement of the TCTcharacteristic can be achieved even if the thickness of the metal plateis made as thick as 0.6 mm or more, and further, 0.8 mm or more.

The circuit substrate of this embodiment is effective as a circuitsubstrate for mounting a semiconductor element of a semiconductordevice. Further, the circuit substrate of this embodiment is suitablealso for a semiconductor device on which a plurality of semiconductorelements are mounted. In the circuit substrate according to thisembodiment, since the shortest distance P between the metal plates canbe made narrow, it is possible to reduce a size of a semiconductordevice on which the same number of semiconductor elements are mounted.Therefore, the semiconductor device can be made smaller. Making thesemiconductor device smaller leads to improvement of a power density.

Next, a manufacturing method of a circuit substrate will be described.The manufacturing method of the circuit substrate according to theembodiment is not limited in particular, but the following method iscited as a method for obtaining the circuit substrate efficiently.

First, a ceramic substrate 2 is prepared. For example, in a case where asilicon nitride substrate is prepared as the ceramic substrate 2, byusing a brazing material containing Ti as a brazing material, a TiNphase (titanium nitride phase) can be formed. The TiN phase contributesto improvement of a bonding strength.

Further, metal plates 3, 4 are prepared. Regarding the metal plates 3,4, plain plates may be bonded and patterned by etching, or metal plateshaving been patterned in advance may be bonded.

Further, the brazing material is prepared for forming bonding layers 5,7. As the brazing material, a brazing material containing an elementapplicable to the bonding layers 5, 7 can be used. For example, in acase of the brazing material constituted by Ag, Cu, and Ti, preferableis a range where Ag is contained 40 to 80 mass %, Cu is contained 15 to45 mass %, Ti is contained 1 to 12 mass %, and Ag+Cu+Ti=100 mass %.Further, in a case where In and Sn are added, it is preferable that atleast one element selected from In and Sn are added in a range of 5 to20 mass %. In a case where C is added, it is preferable that C is addedin a range of 0.1 to 3 mass %.

In a case of a brazing material containing Ag, Cu, Ti, Sn (or In), andC, preferable is a range where Ag is contained 40 to 80 mass %, Cu iscontained 15 to 45 mass %, Ti is contained 1 to 12 mass %, Sn (or In) iscontained 5 to 20 mass %, C is contained 0.1 to 2 mass %, andAg+Cu+Ti+Sn (or In)+C=100 mass %. Though compositions of the brazingmaterial using Ti were explained here, part or all of Ti may be replacedby Zr or Hf.

In order to form bonding layers 5, 7 which contain Ag, Cu, and at leastone element selected from Ti, Zr, and Hf, it is preferable to use abrazing material containing respective elements. Among active metalsselected from Ti, Zr, and Hf, Ti is preferable. When a silicon nitridesubstrate is used as the ceramic substrate 2, Ti can form TiN to formsolid bonding. It is also effective to add at least one element selectedfrom In (indium), Sn (tin), and C (carbon) to those brazing materials.

In order to make an average value of Vickers hardnesses of theprotruding portions 6, 8 be 250 or less, it is preferable to set Ag/Cuto 2.4 or less, and further, 2.1 or less in mass ratio. Further, it ismore preferable to set Ag/Cu to 1.2 or more and 1.7 or less in massratio. Controlling Ag/Cu makes it easy to control the average value ofVickers hardnesses of the protruding portion to 250 or less.

When an Ag—Cu—Ti brazing material is used, it is possible to bond themetal plates 3, 4 to the ceramic substrate 2 by using an eutectic ofAg—Cu. The Ag—Cu eutectic contains 72 mass % Ag and 28 mass % Cu. Thus,a mass ratio Ag/Cu is 2.57. A mass ratio Ag/Cu of a normal Ag—Cu—Tibrazing material is approximately 2.57. Meanwhile, the Ag—Cu eutectic isa hard crystal. Increase of the Ag—Cu eutectic makes a bonding layerhard. Thus, hardnesses of the protruding portions 6, 8 also become high.Further, if the Ag—Cu eutectics are not formed uniformly, variation ofthe Vickers hardnesses also becomes large.

By setting the mass ratio of Ag/Cu to 2.4 or less, a Cu amount can bemade larger than a eutectic composition. By increasing the Cu amount,the hardnesses of the protruding portions 6, 8 can be reduced. Further,it is also effective to make a brazing material contain at least oneelement selected from In, Sn, and C.

By the brazing material using In or Sn, bonding at a low temperature ispossible (a melting point of the brazing material is lowered), and aresidual stress of a bonded body can be decreased. Decrease of theresidual stress is effective in improvement of heat cycle reliability ofthe bonded body. When a content of at least one element selected from Inand Sn is less than 5 mass %, sufficient effect of addition is difficultto be obtained. Meanwhile, the content over 20 mass % leads toapprehension that the bonding layer becomes too hard.

C is effective in reducing variation of hardnesses of a bonding layer. Ccan control fluidity of a brazing material. By adding 0.1 to 2 mass % Cto the brazing material, fluidity can be suppressed. Therefore,variation of hardnesses of the bonding layer can be made smaller. Whenan added amount of C is less than 0.1 mass %, an effect of addition isinsufficient. Meanwhile, when the added amount of C is over 2 mass %,the bonding layer may become too hard.

Ti is preferable to be added in a range of 1 to 12 mass %, and further,5 to 11 mass %. Ti reacts to nitrogen of a silicon nitride substrate,for example, to form a TiN (titanium nitride) phase. By forming the TiNphase, a bonding strength can be improved. Bonding strengths (peelstrengths) of the metal plates 3, 4 can be made as high as 17 kN/m ormore, and further, 20 kN/m or more.

When a mass ratio of Ag/Cu is 2.4 or less, a Ti amount is preferable tobe set to 5 mass % or more. Increasing the Cu amount leads to reductionof the amount of the Ag—Cu eutectic, by which an alleviation effect of aheat stress can be suppressed from becoming insufficient as a resultthat the protruding portion becomes too hard. Meanwhile, increasing theTi amount makes it easy to form an Ag—Ti crystal and Cu—Ti crystal at atime of heating the brazing material. Melting points of the Ag—Ticrystal and of the Cu—Ti crystal are close to each other. Therefore,setting the mass ratio Ag/Cu to 2.4 or less can prevent melting pointsof brazing material structures from varying due to increase of Cu. Bythis effect, it is possible to reduce variation of Vickers hardnesses.Further, though a case where Ti is used is explained here, the sameapplies to a case where Zr or Hf is used.

Next, there is prepared a brazing material paste in which theabove-described components of the brazing material are mixedhomogeneously. A metal component in the brazing material paste is set to60 to 95 mass % or less. The remaining 5 to 40 mass % is accounted forby a resin binder, an organic solvent, or the like. By setting the metalcomponent to 90 mass % or more, unevenness of coating can be made small.

Next, the brazing material paste is applied to first and second surfacesof the ceramic substrate 2, the metal plate 3 is disposed on a brazingmaterial paste layer on the first surface, and the metal plate 4 isdisposed on a brazing material paste layer on the second surface. Notethat a method may be one where the brazing material paste is appliedonto the metal plates 3, 4 and the ceramic substrate 2 is disposedthereon. Regions to become the protruding portions 6, 8 may be small.Further, the protruding portions 6, 8 may be formed by an etchingprocess as will be described later.

By the above-described process is formed a circuit substrate of fivelayer structure of metal plate 3/blazing material paste layer (bondinglayer 5)/ceramic substrate 2/blazing material paste layer (bonding layer7)/metal plate 4.

Next, a heat bonding process is carried out. A heating temperature is ina range of 700 to 900° C., for example. Further, it is preferable tocool rapidly at a cooling rate of 5° C./min or more after heat bonding.A rapid cooling process is preferable to be carried out to a temperatureof equal to or lower than a freezing point temperature of the bondinglayers 5, 7. By carrying out the rapid cooling process, the bondinglayers 5, 7 can be solidified early. As a result that the brazingmaterial melted by the heating process is solidified early and becomesthe bonding layers 5, 7, variation of Vickers hardnesses can be reduced.

Next, an etching process is carried out as necessary. Patterning of themetal plates 3, 4 is carried out by the etching process. Side surfacesof the metal plates 3, 4 may be inclined by the etching process.Further, protruding lengths and thicknesses of the protruding portions6, 8 may be adjusted by the etching process. The process to etch themetal plates 3, 4 and the etching process to arrange shapes of theprotruding portions 6, 8 may be the same process or different processes.

With the above-described manufacturing processes, the circuit substratecan be manufactured efficiently. Further, it is possible to mount thenecessary number of semiconductor elements on the circuit substrate tomanufacture a semiconductor device.

EXAMPLES Examples 1 to 13, Comparative Examples 1 to 4

Ceramic substrates 1 to 4 having characteristics shown in Table 1 areprepared as ceramic substrates, and brazing materials shown in Table 2are prepared as brazing materials. A quality of material in Table 1means a material of a major component of the ceramic substrate. Forexample, “silicon nitride” indicates a ceramic substrate whose maincomponent is silicon nitride. “Alumina zirconia” indicates a ceramicsubstrate whose main component is aluminum oxide containing ZrO₂(zirconium oxide). “Alumina” indicates a ceramic substrate whose maincomponent is aluminum oxide.

TABLE 1 Substrate Size Three- (Vertical Length Point (mm) × HorizontalBending Heat Quality of Length (mm) × Strength Conductivity MaterialThickness (mm)) (Mpa) (W/m · K) Ceramic Silicon 50 × 40 × 0.32 600 90Substrate 1 Nitride Ceramic Silicon 50 × 40 × 0.25 700 90 Substrate 2Nitride Ceramic Alumina 50 × 40 × 0.32 520 20 Substrate 3 ZirconiaCeramic Alumina  50 × 40 × 0.635 400 25 Substrate 4

TABLE 2 Brazing Material Component Mass Ratio (mass %) (Ag/Cu) BrazingMaterial 1 Ag(76), Cu(23), Ti(1) 3.3 Brazing Material 2 Ag(60), Cu(25),In(10), Ti(5) 2.4 Brazing Material 3 Ag(50), Cu(24), Sn(12), In(6),Ti(7), 2.1 C(1) Brazing Material 4 Ag(48.5), Cu(36), Sn(7), Ti(8), 1.3C(0.5) Brazing Material 5 Al(99.5), Si(0.5) —

A front metal plate was disposed on a first surface of the ceramicsubstrate via a first brazing material, a back metal plate was disposedon a second surface of the ceramic substrate via a second brazingmaterial, heat bonding was carried out at 700 to 900° C., and coolingwas carried out at a cooling rate of 5° C./min or more after heatbonding. Thereafter, etching was carried out to adjust a protrudinglength L (μm) and a thickness H (μm) of a protruding portion, and ashortest distance P between the metal plates. Thereby, circuitsubstrates of Examples 1 to 13 and of Comparative Examples 1 to 4 werefabricated. Table 3 shows the ceramic substrates, first brazingmaterials, and second brazing materials which were used in the circuitsubstrates of Examples 1 to 13. Tables 3, 4 show other characteristicsof the circuit substrates of Examples 1 to 13 and Comparative Examples 1to 4. Note that copper plates were used as the metal plates of Examples1 to 3, Examples 5 to 13, and Comparative Examples 1 to 4. An aluminumplate was used as the metal plate of Example 4.

TABLE 3 Metal Plate Back Metal Front Metal Plate Plate (Vertical Length(Vertical (mm) × Length (mm) × Horizontal Horizontal First And Length(mm) × Length (mm) × Ceramic Second Brazing Thickness P ThicknessSubstrate Materials (mm)) Number (mm) (mm)) Example 1 Ceramic Brazing 20× 35 × 0.8 2 1.1 45 × 35 × 0.6 Substrate 1 Material 2 Example 2 CeramicBrazing 20 × 35 × 0.8 2 1.0 45 × 35 × 0.8 Substrate 1 Material 3 Example3 Ceramic Brazing 10 × 35 × 0.8 3 1.0 45 × 35 × 0.8 Substrate 1 Material4 Example 4 Ceramic Brazing 20 × 35 × 0.8 2 1.1 45 × 35 × 0.6 Substrate1 Material 5 Example 5 Ceramic Brazing 20 × 35 × 0.8 2 1.1 45 × 35 × 0.6Substrate 2 Material 2 Example 6 Ceramic Brazing 20 × 35 × 0.8 2 1.0 45× 35 × 0.8 Substrate 2 Material 3 Example 7 Ceramic Brazing 10 × 35 ×0.8 3 1.0 45 × 35 × 0.8 Substrate 2 Material 3 Example 8 Ceramic Brazing10 × 35 × 0.8 3 1.0 45 × 35 × 0.8 Substrate 2 Material 4 Example 9Ceramic Brazing 10 × 35 × 0.8 3 0.9 45 × 35 × 0.8 Substrate 2 Material 4Example 10 Ceramic Brazing 10 × 35 × 0.8 2 1.1 45 × 35 × 0.6 Substrate 3Material 2 Example 11 Ceramic Brazing 20 × 35 × 0.9 2 0.9 45 × 35 × 0.8Substrate 2 Material 3 Example 12 Ceramic Brazing 20 × 35 × 1.0 2 0.9 45× 35 × 0.8 Substrate 2 Material 4 Example 13 Ceramic Brazing 20 × 35 ×1.0 2 0.9 45 × 35 × 0.8 Substrate 2 Material 4 Comparative CeramicBrazing 20 × 35 × 0.8 2 1.1 45 × 35 × 0.6 Example 1 Substrate 1 Material1 Comparative Ceramic Brazing 20 × 35 × 0.8 2 1.1 45 × 35 × 0.6 Example2 Substrate 1 Material 2 Comparative Ceramic Brazing 20 × 35 × 0.8 2 1.145 × 35 × 0.6 Example 3 Substrate 3 Material 1 Comparative CeramicBrazing 20 × 35 × 0.8 2 1.2 45 × 35 × 0.6 Example 4 Substrate 4 Material1

A Vickers hardness HV of an arbitrary protruding portion of the obtainedcircuit substrate was measured. In measurement of the Vickers hardness,TI 950 TriboIndenter manufactured by Hysitron, Inc. was used as a nanoindenter, and a Berkovich type diamond triangular pyramid indenter wasused as an indenter. FIG. 4 illustrates a load condition. In FIG. 4, ahorizontal axis indicates a time (second) and a vertical axis indicatesa load (A). A maximum load is 1500 μN and the load was increasedgradually over 50 seconds. An indentation depth at this time wasmeasured to obtain a nano indentation hardness HIT. The nano indentationhardness HIT was converted into a Vickers hardness HV by using thefollowing conversion formula.Vickers hardness HV(kgf/mm²)=76.23×nano indentation hardnessHIT(GPa)+6.3

Vickers hardnesses HV of arbitrary 10 places were calculated and anaverage value thereof was obtained. Further, a difference(HV_(max)−HV_(min)) between a maximum value and a minimum value of theVickers hardnesses of 10 places was obtained. Results are shown in Table4.

TABLE 4 Protruding Portion Angle θ made by Side Protruding AverageSurface and Inner Length Thickness Protruding Value of HV_(max) −Surface of Metal Plate (μm) (μm) Length/Thickness HV HV_(min) (degree)Example 1 60 20 3.0 240 70 50 Example 2 40 20 2.0 200 50 60 Example 3 2525 1.0 160 40 70 Example 4 15 30 0.5 120 60 70 Example 5 25 10 2.5 24070 80 Example 6 30 15 2.0 200 50 70 Example 7 15 10 1.5 200 50 60Example 8 20 20 1.0 160 40 50 Example 9 15 15 1.0 160 40 45 Example 1014 20 0.7 210 85 50 Example 11 24 20 1.2 150 40 55 Example 12 22 16 1.4120 30 55 Example 13 27 17 1.6 110 20 55 Comparative 4 20 0.2 300 80 50Example 1 Comparative 100 20 5.0 240 70 50 Example 2 Comparative 40 202.0 280 90 50 Example 3 Comparative 40 25 1.6 280 90 50 Example 4

Next, regarding the circuit substrates according to Examples 1 to 13 andComparative Examples 1 to 4, bonding strengths of arbitrary metal platesand TCT characteristics were measured. The bonding strength of the metalplate was obtained as a peel strength. More specifically, a metalterminal of 1 mm in width is bonded to the metal plate and pulled in aperpendicular direction, to measure the peel strength.

TCTs were carried out under two kinds of conditions. In a test 1, withone cycle being −40° C.×holding for 30 minutes→room temperature×holdingfor 10 minutes→125° C.×holding for 30 minutes→room temperature×holdingfor 10 minutes, existence/absence of a defect of the circuit substrateafter 3000 cycles was measured. In a test 2, with one cycle being −40°C.×holding for 30 minutes→room temperature×holding for 10 minutes→250°C.×holding for 30 minutes→room temperature×holding for 10 minutes,existence/absence of a defect of the circuit substrate after 3000 cycleswas measured. Existence/absence of the defect of the circuit substratewas evaluated by obtaining an area of crack occurrence between theceramic substrate and the metal plate through scanning acoustictomograph (SAT). The area of crack occurrence was evaluated as an indexeta. Regarding the eta, 100% indicates “without a crack” and 0%indicates “cracks occurring extensively”. Results thereof are shown inTable 5.

TABLE 5 Peel Strength η (%) (kN/m) Test 1 Test 2 Example 1 24 100 96Example 2 26 100 100 Example 3 30 100 100 Example 4 18 100 95 Example 528 100 98 Example 6 28 100 100 Example 7 28 100 100 Example 8 30 100 100Example 9 30 100 100 Example 10 17 100 90 Example 11 29 100 100 Example12 31 100 100 Example 13 31 100 100 Comparative 24 80 20 Example 1Comparative 24 100 80 Example 2 Comparative 24 100 70 Example 3Comparative 17 100 10 Example 4

As is known from Table 5, in the circuit substrates according toExamples 1 to 13, the peel strengths were high and the areas of crackoccurrence were small (index η was large). As in the test 1, when atemperature of a high temperature side was 125° C., the areas of crackoccurrence did not virtually exist. Meanwhile, as in the test 2, whenthe temperature was 250° C., the areas of crack occurrence became large.When durability at a high temperature is viewed, it is found that thecircuit substrates according to Examples 1 to 13 are excellent.

As in Comparative Example 1, in a case of L/H of less than 0.5, the areaof crack occurrence was large. As in Comparative Example 2, also in acase of L/H of over 3.0, the area of crack occurrence was large. Fromthe above, it is found that a heat stress of the protruding portionbecomes large when the temperature is high. Further, as in ComparativeExample 3, also in a case of high Vickers hardness of the protrudingportion, the area of crack occurrence became large.

In a case that the strength of the ceramic substrate was low as inComparative Example 4, the area of crack occurrence was large inparticular. From the above, it is found that control of L/H of theprotruding portion and the Vickers hardness is an effective technologyfor a high-strength substrate of 500 MPa or more in three-point bendingstrength.

Hereinabove, several embodiments of the present invention wereexemplified, but these embodiments have been presented by way ofexamples only, and are not intended to limit the scope of theinventions. The novel embodiments described herein can be implemented ina variety of other forms, furthermore, various omissions, substitutionsand changes may be made without departing from the spirit of theinventions. Those embodiments and modification examples thereof fallwithin the range and basic gist of the invention and fall within therange of the invention described in what is claimed is and itsequivalent. Further, the aforementioned respective embodiments can beimplemented by being combined with each other.

What is claimed is:
 1. A circuit substrate comprising: a ceramicsubstrate including a first surface and a second surface; a first metalplate bonded to the first surface via a first bonding layer; and asecond metal plate bonded to the second surface via a second bondinglayer, wherein the first bonding layer has a first portion protrudingfrom between the first surface and the first metal plate and extendingonto the first surface, wherein the second bonding layer has a secondportion protruding from between the second surface and the second metalplate and extending onto the second surface, wherein at least one of thefirst bonding layer or the second bonding layer contains Ag, Cu, and Ti,wherein a three-point bending strength of the ceramic substrate is 500MPa or more, wherein at least one of a ratio L1/H1 of a length L1 (μm)of the first portion to a thickness H1 (μm) thereof or a ratio L2/H2 ofa length L2 (μm) of the second portion to a thickness H2 (μm) thereof is0.5 or more and 3.0 or less, and wherein at least one of an averagevalue of first Vickers hardnesses of 10 places of the first portion oran average value of second Vickers hardnesses of 10 places of the secondportion is 90 or more and 250 or less.
 2. The circuit substrateaccording to claim 1, wherein at least one of the average value of thefirst Vickers hardnesses or the average value of the second Vickershardnesses is 90 or more and 230 or less.
 3. The circuit substrateaccording to claim 1, wherein at least one of the average value of thefirst Vickers hardnesses or the average value of the second Vickershardnesses is 100 or more and 170 or less.
 4. The circuit substrateaccording to claim 1, wherein at least one of the ratio L1/H1 and theratio L2/H2 is 1.0 or more and 2.0 or less.
 5. The circuit substrateaccording to claim 1, wherein the ceramic substrate is a silicon nitridesubstrate.
 6. The circuit substrate according to claim 1, wherein atleast one of the first metal plate or the second metal plate is a copperplate, wherein a thickness of the copper plate is 0.6 mm or more, andwherein a thickness of the ceramic substrate is 0.4 mm or less.
 7. Thecircuit substrate according to claim 1, wherein at least one of thelength L1 or the length L2 is 40 μm or less.
 8. The circuit substrateaccording to claim 1, wherein in a cross section of the first metalplate along a thickness direction thereof, a side surface of the firstmetal plate is inclined toward the edge of an outer surface of the firstmetal plate, and wherein in a cross section including a thicknessdirection of the second metal plate a side surface of the second metalplate is inclined toward the edge of an outer surface of the secondmetal plate.
 9. The circuit substrate according to claim 1, wherein atleast one of a difference between a maximum value and a minimum value ofthe first Vickers hardness or a difference between a maximum value and aminimum value of the second Vickers hardness is 50 or less.
 10. Thecircuit substrate according to claim 1, wherein each of the first andsecond bonding layers contains Ag, Cu, and Ti.
 11. The circuit substrateaccording to claim 10, wherein at least one of the first bonding layeror the second bonding layer further contains at least one elementselected from the group consisting of In, Sn, and C.
 12. The circuitsubstrate according to claim 1, comprising a plurality of the firstmetal plates.
 13. The circuit substrate according to claim 1, comprisinga plurality of the first metal plates, wherein a shortest distancebetween one of the first metal plates and another one of the first metalplates is 1.1 mm or less.
 14. A semiconductor device comprising thecircuit substrate according to claim 1; and a semiconductor elementmounted on the circuit substrate.
 15. The semiconductor device accordingto claim 14, comprising a plurality of the semiconductor elements. 16.The circuit substrate according to claim 1, wherein the ceramicsubstrate is a silicon nitride substrate having a heat conductivity of80 W/mK and a thickness of 0.30 mm or less.
 17. The circuit substrateaccording to claim 1, wherein at least one of the first metal plate orthe second metal plate is a copper plate having a thickness of 0.6 mm ormore.
 18. The circuit substrate according to claim 17, comprising aplurality of the first metal plates, wherein the shortest distancebetween one of the first metal plates and another one of the first metalplates is 1.1 mm or less.
 19. The circuit substrate according to claim1, wherein the mass ratio of cupper to silver is 2.4 or less in at leastone of the first portion or the second portion.
 20. The circuitsubstrate according to claim 19, wherein at least one of the first metalplate or the second metal plate is a copper plate having a thickness of0.6 mm or more.
 21. The circuit substrate according to claim 20,comprising a plurality of the first metal plates, wherein a shortestdistance between one of the first metal plates and another of the firstmetal plates is 1.1 mm or less.